How would you explain the process of an action potential based on the given graph?

The graph's two axes represent the membrane potential (mV) and the time (ms). We can see four phases: the resting potential, repolarisation, depolarisation and then rest potential again. It all starts with a stimulus that excites the neuron. This electric stimulation must reach a threshold (-50 mV) because otherwise no action potential will take place as it is an all-or-none event! In resting potential, the potassium channels are open and K+ flows freely based on the concentration and electrical gradient. At -50 mV, K+ channels close and voltage-gated sodium channels open, so Na+ flows into the cell. The cell therefore becomes more positive on the inside (depolarises) and soon reaches +40 mV, where Na+ channels shut and K+ channels open and the cell repolarises, hyperpolarises even. However, there is an ion imbalance which prevents the next action potential from taking place. This imbalalnces is reversed by the sodium-potassium pump, which pushes Na+ out of the cell and draws K+ inside. As we reached the original state, the resting potential, another action potential can finally happen.